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Atrial natriuretic peptide molecule. Computer model showing the structure of a molecule of the hormone atrial natriuretic peptide (ANP). ANP is a powerful vasodilator (dilates blood vessels) that is secreted by muscle cells (atrial myocytes) in the upper chambers (atria) of the heart. It is involved in the homeostatic control of body water, sodium, potassium and fat tissue (adipose tissue). It is released in response to high blood pressure and acts to reduce the water, sodium and adipose loads on the circulatory system, thereby reducing blood pressure.